Automatic transmissions, particularly for motor vehicles, in the state of the art, include planetary gearsets that are shifted by way of friction or shifting elements, such as clutches and brakes, and a starting element, such as a hydrodynamic torque converter or a fluid clutch, subjected to a slip effect and optionally provided with a lockup clutch.
Within the scope of the Applicant's DE 101 15 983 A1 is described a multi-speed transmission with an input shaft connected to a front-mounted gear train and a rear-mounted gear train connected to an output shaft, and a maximum of seven shifting elements, the selective shifting of which implements at least seven forward gears without range shifts. The front-mounted gear train is composed of a shiftable or non-shiftable planetary gearset, or a maximum of two non-shiftable planetary gearsets that are coupled to each other. The rear-mounted gear train is configured as a two-carrier four-shaft transmission with two shiftable planetary gearsets and features four free shafts. The first free shaft of this two-carrier four-shaft transmission is connected to the first shifting element, the second free shaft to the second and third shifting element, the third free shaft to the fourth and fifth shifting element, and the fourth free shaft is connected to the output shaft. For a multi-speed transmission with a total of six shifting elements, it is proposed according to the invention to connect the third free shaft or the first free shaft of the rear-mounted gear train additionally to a sixth shifting element. For a multi-speed transmission with a total of seven shifting elements, it is proposed according to the invention to connect the third free shaft additionally to a sixth shifting element, and the first free shaft additionally to a seventh shifting element.
Several other multi-speed transmissions are also known from the Applicant's DE 101 15 995 A1, which are provided with four shiftable planetary gearsets that are coupled to each other and six or seven non-positive shifting elements, by way of whose selective engagement a rotational speed of an input shaft of the transmission can be transferred to an output shaft of the transmission such that nine or eleven forward gears and at least one reverse gear can be engaged. Depending on the gearbox arrangement, two or three shifting elements are engaged in each gear, such that when shifting from one gear into the next higher or next lower gear in order to avoid group shifts, only one engaged shifting element is disengaged, and one shifting element that was previously disengaged is engaged.
In addition, the unpublished patent application DE 10 2005 002 337.1 of the Applicant proposes a multi-speed transmission with an input shaft, an output shaft, four individual planetary gearsets that are coupled to each other, and five shifting elements, with which eight forward gears can be shifted into without range shifts, that is, in such a way, that when shifting from a forward gear into the next higher or lower forward gear, only one of the previously engaged shifting elements is disengaged and only one of the previously disengaged shifting elements is engaged. This muiti-speed transmission also features one reverse gear. In all forward gears and in the reverse gear, three shifting elements are engaged at any one time. With regard to the kinematic coupling of the four planetary gearsets to each other and to the input and output shafts, it is provided that a carrier of the fourth planetary gearset and the input shaft are connected to each other and form a first shaft of the transmission; a carrier of the third planetary gearset and the output shaft are connected to each other and form a second shaft of the transmission; a sun gear of the first planetary gearset and a sun gear of the fourth planetary gearset are connected to each other and form a third shaft of the transmission; a ring gear of the first planetary gearset forms a fourth shaft of the transmission; a ring gear of the second planetary gearset and a sun gear of the third planetary gearset are connected to each other and form the fifth shaft of the transmission; a carrier of the first planetary gearset and a ring gear of the third planetary gearset are connected to each other and form a sixth shaft of the transmission; a sun gear of the second planetary gearset and a gear of the fourth planetary gearset are connected to each other and form a seventh shaft of the transmission; and a carrier of the second planetary gearset forms an eighth shaft of the transmission. Regarding the kinematic coupling of the five shifting elements to the four planetary gearsets and to the input and output shafts, it is provided that the first shifting element is arranged within the power flow between the third shaft and a housing of the transmission, the second shifting element, between the fourth shaft and the housing of the transmission, the third shifting element, between the first and fifth shafts, the fourth shifting element either, between the eighth and second shaft or between the eighth and sixth shaft, and the fifth shifting element is either between the seventh and fifth shafts, between the seventh and eighth or between the fifth and eighth shaft.
Automatically shiftable motor vehicle transmissions in planetary design have in general already been described many times in the state of the art and are subjected to constant further development and improvement. These transmission should therefore feature a sufficient number of forward gears, as well as one reverse gear, and transmission ratios that are very well suited for motor vehicles, with a high total transmission ratio spread, and with favorable progressive ratios. In addition, they should enable a high startup gear ratio in forward direction and contain a direct gear for use in both passenger cars and commercial vehicles. In addition, these transmissions should not have a complicated construction and should require a small number of shifting elements and avoid double shifting within a sequential shifting method so that only one shifting element is activated at one time when shifting within defined gear ranges.
It is an object of the invention to propose a multi-speed transmission of the type discussed above with at least eight shiftable forward gears, without range shifting, and at least one reverse gear, in which the lowest possible number of shifting elements is required with the use of a total of four planetary gearsets. In addition, the transmission should feature a large ratio spread with comparatively harmonic gear stepping, and at least in the main driving gears, a favorable degree of efficiency, that is, comparatively little drag and gearing loss.